The inventor of the present invention has been producing organic acids (for example, acrylic acid) by a gas-phase catalytic oxidation reaction. Since a large quantity of waste heat is generated in a large scale chemical plant as well as in a chemical plant for carrying out a gas-phase catalytic oxidation reaction, efforts have been made to use the waste heat as efficiently as possible by introducing heat utilization techniques.
For example, Japanese Patent Application Laid-Open (JP-A) H7-65818 discloses a technique concerning to a method for recovering waste heat from cooling water in a fuel cell equipment. According to this technique, steam is obtained from cooling water used for heat removal at the time of a fuel cell generating power, and then the steam is supplied to a waste gas turbine power generator, and at the same time the steam is supplied to an absorption type refrigerator to obtain cold water. JP-A-2002-266656 discloses a technique concerning to a gas turbine cogeneration system. According to this technique, warm water produced by using waste gas passed through a waste heat recovery boiler is utilized to a driving heat source for an adsorption type refrigerator. In the technique of JP-A-2002-266656, cold water is used for cooling air sucked into the turbine.
Waste heat is also utilized in an organic compound production plant. JP-A-S57-93946 discloses a technique of driving an absorption type refrigerator or the like by using a heat source of 90° C. or less, which obtained in an ammoxidation process at the temperature of about from 100° C. to 130° C., as a driving source. JP-A-H5-17377 discloses a technique of utilizing heat on production of styrene.
Further, waste heat utilization in a waste incinerator, although not in a chemical plant, has been discussed. JP-A-H11-83232 discloses a technique of producing steam by a boiler using combustion gas generated in a waste incinerator, driving a turbine of a power generator by the steam to generate electric power, at the same time using a part of low temperature steam exhausted from the turbine as a driving heat source for a lithium bromide absorption type refrigerator, and using a part of high temperature steam at the inlet of the turbine as a driving heat source for an ammonia absorption type refrigerator.
However, the techniques described in the above-mentioned JP-A-H7-65818, JP-A-2002-266656, JP-A-S57-93946, JP-A-H5-17377, and JP-A-H11-83232 are not methods for utilizing waste heat in an organic acid production by a gas-phase catalytic oxidation reaction. Thus, it cannot say that it is appropriate to apply these techniques to an organic acid production plant as they are.
In the case of carrying out a gas-phase catalytic oxidation reaction, it is required to supply a raw material in gas phase. For example, propylene liquid as a raw material is gasified through an evaporator and supplied to a reactor in the case of acrylic acid production. When a raw material such as propylene or the like is subjected to a gas-phase catalytic oxidation with molecular oxygen-containing gas in the presence of a gas-phase catalytic oxidation reaction catalyst, produced gas containing an aimed organic acid and by-products as well is obtained and at the same time a large quantity of reaction heat is generated. Generally, the reaction heat is recovered by heat exchange.
In the successive step, the gas produced by the gas-phase catalytic oxidation is led to an absorbing column of an organic acid and contacted with a solvent (generally water is used) for collecting the organic acid to be cooled, absorbed, and collected. The obtained solution containing the organic acid and by-products is successively refined by a method such as distillation and crystallization to obtain the refined organic acid.
With respect to utilization of reaction heat generated in an organic acid production by such gas-phase catalytic oxidation reaction, the inventor has already been examining. For example, the inventor applied an invention for a patent of JP-A-2003-73327 which discloses a method of recovering heat (including reaction heat) generated in an acrylic acid production plant as steam and using the heat as heat energy, mechanical energy, or electric energy. The inventor also applied an invention for a patent of JP-A-2003-268011 which discloses a method of using steam or cold liquid, which is generated in a production process of acrylic acid, in a poly(acrylic acid) production plant.
Recently, collecting efficiency in an absorbing column of an organic acid has been improved to obtain high concentration acrylic acid aqueous solution. Conventionally, as a post-process of collecting acrylic acid, refining has been carried out by steps which are combined of dehydration distillation, distillation for removing high boiling point impurities, acrylic acid recovery distillation, aldehyde-removal distillation and the like, and low pressure steam recovered in a reaction step and the like has been utilized for the distillation steps. However, when the high concentration acrylic acid aqueous solution comes to be obtained, the number of the succeeding distillation steps and time taken for the steps can be saved, and used amount of the steam recovered in an acrylic acid production plant tends to be decreased accordingly.
In an organic acid ester production step or in a crystallization step which is involved in the refining steps, a large quantity of cold water (brine) is required. To produce the cold water, a refrigerator has been driven by using commercial electric power or by generating electric power by rotating a turbine of a power generator by steam. However, in the first case, cost becomes high, and in the other case, electric power generation efficiency cannot be high. The above-mentioned JP-A-2003-268011 implies use of an absorption type refrigerator, however, it is not mentioned how to use the refrigerator.
Accordingly, the purpose of the invention is to find a more specific and practical method of an effective utilization of reaction heat than methods disclosed in JP-A-2003-73327 and JP-A-2003-268011, and to provide a production method of an organic acid that is favorable to global environmental preservation and saving production cost.